Multiple smoke flows in one domain, with different colors
You can use an Attribute node with color as a keyword to read color information from smoke (use colored flow objects).
Available keywords are:


color
density
flame
heat *
velocity *


They are now all defined in /addons/cycles/kernel/kernel_types.h though dunno if the last 2 are yet usable (maybe in some test builds) - edit - these attributes are usable from version 2.78 up - from left to right: smoke render, heat map, velocity vectors:



Smoke sim from benu: